1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device useful in display applications, and to a process for manufacturing the same. More specifically, the invention relates to an organic EL device which prevents the infiltration of moisture from the external environment and exhibits an excellent luminous efficiency over an extended period of time, and to a process for manufacturing the same.
2. Background of the Related Art
Recently, in connection with display applications, research is actively being carried out on organic EL devices that use self-luminescent organic EL elements. There is an expectation that organic EL devices will achieve high luminance and luminous efficiency, the reason being that such devices can manifest a high current density at low voltage. The practical use of high-definition multicolor-emitting organic EL devices capable of full-color display is especially anticipated in the technical field of displays.
Key challenges for the practical implementation of organic EL devices as color displays are achieving a high degree of definition, and also imparting the devices with long-term stability, including color reproducibility. However, a drawback of multicolor-emitting organic EL devices is that, when driven for a fixed length of time, a dramatic falloff in the emission characteristics (current-luminance characteristics) occurs.
A typical cause of this decline in the emission characteristics is the growth of dark spots. Here, the term ‘dark spot’ refers to an emission point defect. Such dark spots are thought to arise as the oxidation or aggregation of material in the component layers of an organic EL element proceeds under the effects of oxygen or moisture within the device during driving and storage. The growth of dark spots proceeds not only during power is on but also during storage. In particular, the growth of dark spots is thought to be (1) accelerated by oxygen or moisture present in the external environment around the element, (2) affected by oxygen or moisture present as adsorbed matter within the component layers, and (3) influenced by moisture already adsorbed onto components that are used in device manufacture or by the infiltration of moisture at the time of manufacture. As such growth continues, the dark spots spread throughout the light-emitting face of the organic EL device.
Methods in which the organic EL element is sealed using a metal casing or a glass sheet or in which a drying agent is placed within a space where the organic EL element is sealed have hitherto been carried out as the means for preventing moisture infiltration into the component layers of the organic EL element. However, to take advantage of the light weight and thin profile which are distinctive features of organic EL devices, technology that seals by means of a thin-film without using a drying agent has been attracting attention.
Thin films that are used for sealing include silicon nitride and silicon nitride oxide. However, to minimize damage to the emissive layer at the time of film formation from such materials, it is essential to hold the temperature rise at the film-forming surface to not more than the glass transition temperature of the emissive layer. For this reason, the film-forming methods that have been developed in semiconductor processes cannot be employed in organic EL devices, making it impossible to form a thin-film for sealing which has sufficient moisture resistance.
To address this challenge, Patent Document 1 describes, as a sealing thin-film capable of being employed in organic EL devices, a film composed primarily of silicon and silicon nitride that is formed by plasma chemical vapor deposition (CVD). Japanese Patent Application Laid-open No. 2005-285659 (Patent Document 1) discloses that, by having the number of silicons bonded to silicon relative to the number of silicons bonded to nitrogen, as measured by x-ray photoelectron spectroscopy, be at least 0.6 but not more than 2.0, such a film exhibits excellent sealability.
Recently, so-called top-emission structure type devices which, in order to increase the aperture ratio of an active matrix drive organic EL device, output light on the side of the substrate opposite from that where a switching circuit containing thin-film transistors (TFT) and the like has been fabricated, have become prevalent. In this structure, a transparent electrode and a sealing film are formed on the organic EL layer, and the light generated by the organic EL layer is emitted to the exterior through the sealing film. However, because the sealing film of Patent Document 1 contains a considerable amount of silicon-silicon bonds, it has a low visible-light transmittance, and thus cannot be used as the sealing film in a top-emission structure.
The object of the invention is to provide an organic EL device which uses a protective layer having a high visible-light transmittance and excellent moisture resistance, and is thus endowed with long-term stability.
A further object of the invention is to provide a process for manufacturing such an organic EL device.